FIGS. 36 and 37 are a sectional front view and a plan view which show a conventional diode as the background of the present invention, respectively. FIG. 36 is a sectional view taken along the line E--E in FIG. 37. A diode 151 comprises, as a main part, a semiconductor substrate 80 using silicon as a base material. The semiconductor substrate 80 has a P layer 81, an N.sup.- layer 82 and an N.sup.+ layer 83 provided sequentially from an upper main surface to a lower main surface.
An anode electrode 84 is connected to the upper main surface of the semiconductor substrate 80, that is, an exposed surface of the P layer 81, and a cathode electrode 85 is connected to the lower main surface of the semiconductor substrate 80, that is, an exposed surface of the N.sup.+ layer 83. These electrodes 84 and 85 are formed of an electrically conductive metal. Furthermore, life time killers which are crystal defects for promoting the annihilation of carriers as the recombination centers of the carriers are introduced into the semiconductor substrate 80. Thereby the life time of the carrier is controlled.
FIG. 38 is a graph showing a profile of a density of the life time killers introduced into the semiconductor substrate 80. In the conventional diode 151, two kinds of profiles have been known. In a conventional example 1 represented by a curve Pr1 shown in FIG. 38, the life time killers are uniformly introduced over the whole semiconductor substrate 80. Accordingly, the life time of the N.sup.- layer 82 is uniformly controlled.
On the other hand, in a conventional example 2 corresponding to a curve Pr2, the life time killers are selectively introduced into a region of the N.sup.- layer 82 adjacent to a junction interface between the N.sup.- layer 82 and the P layer 81. Consequently, the life time of the region adjacent to the junction interface between the N.sup.- layer 82 and the P layer 81 is locally controlled to be short. A diode corresponding to the conventional example 2 is a device which has been disclosed in the International Conference PCIM '97 (International POWER CONVERSION '97 CONFERENCE NURNBERG, GERMANY Jun. 10-12, 1997).
Immediately after operating conditions are instantaneously changed by the switching operation of an external circuit from a state in which a current flows to a diode in a forward direction to a state in which a reverse bias is applied, a reverse current transiently flows to the diode. FIG. 39 is a graph showing a waveform of a current flowing in the diode in the transient state in relation to both the conventional examples 1 and 2. At a time t0, when a switching operation is performed from a state in which a forward current I.sub.F steadily flows to a state in which a reverse bias is applied, a current starts to be decreased. The current is continuously decreased to have a negative value in a short time. In other words, a reverse current (minus current) flows to the diode.
Even if the switching operation is performed to apply a reverse bias, a depletion layer is not immediately formed in a PN junction between the P layer 81 and the N.sup.- layer 82 due to excess carriers remaining in the vicinity of the PN junction. For this reason, the diode is transiently brought into a conductive state. As a result, the reverse current flows. An increase rate of the reverse current in an initial stage, that is, (an absolute value of) a current decrease rate represented by di/dt in FIG. 39 is defined by the magnitude of an inductance acting as a load in the external circuit. If the inductance is increased, the current decrease rate di/dt is increased. Correspondingly, the reverse current is rapidly increased.
In a process in which the reverse current is increased, a depletion layer is generated at a time tl. The depletion layer is formed in the PN junction as shown in FIG. 40. A front 92 of a depletion layer 91 advances toward the N.sup.+ layer 83 with the passage of a time. Consequently, the depletion layer 91 is enlarged to cover the whole N.sup.- layer 82 shortly.
Returning to FIG. 39, as the depletion layer 91 is generated and grows, a reverse voltage v is generated at the time t1 between the anode electrode 84 and the cathode electrode 85, and then the reverse voltage v is increased to shortly converge on a value of a reverse bias applied from the outside. More specifically, when the depletion layer 91 grows, a reverse voltage blocking capability which is the original function of the diode is recovered. FIG. 39 typically shows only the reverse voltage v related to the conventional example 2.
When the reverse voltage v is increased, the reverse current gradually reduces the speed of the increase, and shortly reaches a peak and is then decreased. As the current decrease rate di/dt is increased, the peak is increased. A value of the peak is referred to as a reverse recovery current I.sub.rr, and is one of parameters for evaluating a reverse recovery characteristic in the diode. The reverse current converges on zero while continuing the decrease. Thus, a transient state, that is, a reverse recovery operation comes to an end, and a steady state in which the reverse voltage v is equal to the reverse bias and the reverse current does not flow is realized.
As the parameter for evaluating the reverse recovery characteristic, an attenuation rate of the reverse recovery current, a di/dt capability and a reverse recovery loss have been known in addition to the above-mentioned reverse recovery current I.sub.rr.
The attenuation rate of the reverse recovery current is defined as a rate of convergence on zero after the reverse current passes through a peak in the graph of FIG. 39. The di/dt capability is a maximum value of the current decrease rate di/dt which can be applied without causing a damage on the diode. Moreover, the reverse recovery loss is a magnitude of a loss caused on the diode in the process of the reverse recovery operation.
If the reverse recovery current I.sub.rr is smaller, it is possible to resist a greater current decrease rate di/dt. Accordingly, a simple relationship is established between the reverse recovery current I.sub.rr and the current decrease rate di/dt. Moreover, the reverse recovery loss is equivalent to a time integral of a product of the reverse current and the reverse voltage v in the graph of FIG. 39. Accordingly, if the magnitude of the reverse recovery current I.sub.rr is smaller and the attenuation of the reverse recovery current is performed more quickly, the reverse recovery loss is more reduced. As a matter of course, it is desirable that the magnitude of the reverse recovery current I, should be smaller, the attenuation of the reverse recovery current should be performed more quickly and the di/dt capability should be larger. Furthermore, it is desirable that the reverse recovery loss should be as small as possible.
In the diode according to the conventional example 1, the life time killers are introduced over the whole semiconductor substrate 80. Therefore, the attenuation of the reverse recovery current is performed quickly as shown in the curve Pr1 of FIG. 39. Consequently, there is an advantage that the reverse recovery loss is small. However, the magnitude of the reverse recovery current I.sub.rr is large. As a result, there has been a problem in that the di/dt capability is small. In addition, there has been a problem in that a forward voltage acting as a significant parameter for evaluating a forward characteristic is high.
In the diode according to the conventional example 2, the life time killers are locally introduced in the vicinity of the PN junction at a higher density than in the conventional example 1. Consequently, the life time of the carriers is controlled to be short in the vicinity of the PN junction. Therefore, the recombination of the excess carriers is performed quickly in the vicinity of the PN junction. For this reason, the formation of the depletion layer 91 is promoted. Thus, the magnitude of the reverse recovery current I.sub.rr is small as shown in the curve Pr2 of FIG. 39. As a result, it is possible to obtain an advantage that the di/dt capability is high.
Furthermore, the life time killers are not introduced into a region of the N.sup.- layer 82 excluding the vicinity of the PN junction. Therefore, the forward voltage is also advantageously low. More specifically, not only the di/dt capability but also the forward characteristic is more improved in the diode according to the conventional example 2 than in the conventional example 1.
However, since the life time killer is not introduced into the region of the N.sup.- layer 82 excluding the vicinity of the PN junction, after the depletion layer 91 is generated during the reverse recovery operation, the depletion layer 91 grows slowly. In the diode according to the conventional example 2, therefore, the attenuation of the reverse recovery current is performed slowly as shown in the curve Pr2 of FIG. 39. As a result, there has been another problem in that a reverse recovery loss is large.
As will be described below, furthermore, both the conventional examples 1 and 2 have had a common problem in that oscillation is easily caused at the last stage of the reverse recovery operation. The oscillation is caused as that of the reverse voltage v in an oscillation region Osc as shown in FIG. 39. Although FIG. 39 illustrates only the reverse voltage v according to the conventional example 2, the oscillation is caused more remarkably in the conventional example 1.
When the time t1 passes in the process of the reverse recovery operation, the depletion layer 91 is generated and then grows as shown in FIG. 40. During this process, the diode 151 can be equivalently represented by a series circuit formed by a capacitor having a pair of electrodes opposed to each other with the depletion layer 91 interposed therebetween and a leak resistor in the depletion layer 91 as shown in FIG. 41. In the process of the reverse recovery operation, accordingly, a series resonance circuit is equivalently constituted by the combination of a capacitance C of the capacitor, a resistance R corresponding to the leak resistor and an inductance L existing in an external circuit. In FIG. 41, a Q value of the resonance circuit is expressed by an equation. An oscillating phenomenon does not occur while the Q value is small.
The capacitance C is defined by a thickness of the deletion layer 91 and a density of the excess carriers, and the resistance R is defined by a leak current in the depletion layer 91 and a recombination current of the excess carrier. As a result, when the depletion layer 91 is enlarged, the capacitance C and the resistance R are changed in a waveform illustrated in a graph of FIG. 42. More specifically, the capacitance C is generated with the generation of the depletion layer 91, and is then increased and is shortly decreased through a peak. Thereafter, the capacitance C converges on zero which is a steady value. On the other hand, the resistance R is generated with the generation of the depletion layer 91, and is then increased steadily and is rapidly raised particularly in the last stage of the reverse recovery operation.
As shown in FIG. 41, as the resistance R and the capacitance C are increased, the Q value is reduced. Accordingly, when the capacitance C converges at zero in the last stage of the reverse recovery operation, the Q value becomes large if the resistance R is not sufficiently large correspondingly. Consequently, the oscillation of the voltage is caused. Based on such a mechanism, the diodes according to the conventional examples 1 and 2 have had a problem in that the voltage oscillation is easily caused at the last stage of the reverse recovery operation. The oscillation is easily caused particularly when the magnitude of the forward current is small and that of the reverse recovery current Ir is large. When the voltage oscillation is caused, the diode acts as a noise source for peripheral circuits.
In the diode according to the prior art, thus, there has been a problem in that it is difficult to simultaneously implement a high di/dt capability, a low reverse recovery loss and a low forward voltage. Furthermore, there has been a problem in that voltage oscillation is easily caused at a certain stage of the reverse recovery operation.